For communication-based train control (CBTC) operation, accurate train position is required to facilitate both automatic and manual moving block operation. In addition, in order to stop a train accurately at stations, with or without platform doors, the precise position of the train is used. Positional precision is also beneficial when parking trains in close proximity in storage and pocket tracks.
To determine position and distance traveled, wheel revolutions are monitored, with distance calculated based on a number of revolutions and wheel diameter. To obtain accurate distance calculations, accurate determination of train wheel diameter is therefore fundamental. It is also desirable to be able to make such determinations dynamically, so that train operation is not slowed or interrupted for wheel diameter measurements.